Fractures are subsurface features that often play a role in the flow of fluids in a reservoir, whether as a conduit or a barrier, and thus, it can be useful to detect and locate them. Fractures may be naturally occurring or artificially induced (e.g., by high pressure injection of fluid into subsurface formations). Detecting and locating fractures is useful in a number of hydrocarbon provinces, including large “tight gas” plays in the western United States, where commercial exploitation makes use of extensive fracturing to overcome low permeability in the reservoir rocks. Knowledge of existing fracture locations in and around a reservoir can be used to more efficiently employ drilling, hydraulic fracture treatments, and production.
Elastic energy generated by a sub-surface drill bit, or some other source, such as a vibratory or piezoelectric downhole source mounted near the drill bit, reflects and scatters off the interfaces and structures surrounding the borehole. Such energy may be recorded by receivers situated along a borehole under construction, along a monitoring borehole, or at the surface of the earth. Elastic energy may be converted to electromagnetic (EM) energy during scattering (via the seismoelectric effect) and recorded by corresponding EM receivers. Elastic and electromagnetic energy can also be generated directly during artificial fracture creation, and may be similarly recorded.
When a drill bit is used as the energy source, well known recording techniques use sensors placed on the drill string and drilling rig to measure, or at least estimate, the actual far-field signature of the source. In most cases the spectral content of signals provided by the drill bit source is more limited than that associated with a controlled source; the resulting image often has lower resolution and more artifacts.
When a controlled source is available on the drill string, vertical seismic profile (VSP) and cross-well analysis/imaging tools can be utilized to produce an image over a corridor or sub-volume of the formation. In this setting, one may achieve pragmatic operational savings by producing cross-well or VSP datasets in conjunction with drilling. In addition, the source energy may sometimes be generated and transmitted into the subsurface in a near pristine open hole environment, without delaying the placement of post-drill casing.
When artificial fracturing provides the energy source, the energy radiates away from the fracture and may be recorded by sensors located in one or more nearby boreholes. Commercial microseismic services use one or more arrays of elastic sensors to triangulate the location of microseisms generated by the fracturing process. However, there has been to date no investigation of how to use seismoelectrically generated electromagnetic radiation created by the fracturing process to separately, or jointly with microseismic recording, detect and locate the fracture.
In any of these cases, sensors used to record the presence of the energy can be deployed in ways that prove unsuitable for conventional subsurface imaging. For example, an array of sensors may be deployed in a sidetrack or nearby well at or near a reservoir interval to record seismic energy generated during horizontal drilling. Reflections from the bed boundaries within the reservoir (i.e., close to the drill bit), may be indistinguishable from direct arrivals. The recorded signal may also be heavily contaminated by guided waves reverberating within reservoir layering. As a result, reliable detection or location of many subsurface features of interest, such as fractures, can be a difficult challenge to overcome.